1. Field of the Invention
The present invention relates, in general, to an optical system to inspect defective parts of a flat panel display (FPD), such as a liquid crystal display (LCD) and, more particularly, to an optical system to attain a uniform and clear image of an object when the object inclined relative to an optical axis of a lens of the optical system is photographed.
2. Description of the Related Art
When defective parts on an FPD, such as an LCD, are inspected, an inspection result of the FPD may be varied according to viewing angles due to a characteristic of a viewing angular field of the FPD. Thus, when the FPD is photographed to inspect the defective parts thereof, it is necessary to photograph the FPD at a variety of angles as well as a vertical position with respect to the FPD.
To solve the above problem, there was proposed an optical system in Korean Patent Laid-Open Publication No. 2003-52528. According to the above Korean Patent Laid-Open Publication, an object was inspected using a common optical system and an inclined optical system. The common optical system photographs a surface of the object at a vertical position above the object. The inclined optical system photographs the surface of the object at an inclined position with respect to the object at a predetermined angle.
In the inclined optical system according to the above-mentioned Korean Patent Laid-Open publication, a CCD camera (charge-coupled device camera) which photographs a picture of the object must be placed to satisfy a specific equation. However, the conventional inclined optical system has a problem in that reference points of distances used in the specific equation are indefinite, thus the CCD camera may not be placed at a desired position. The conventional inclined optical system also has another problem in that it has no pickup unit to photograph the object at a variety of angles.